The commutator and brush type machines available in the prior art have some unique performance characteristics which makes them indispensable over other types of machines such as induction, synchronous, and some eddy current drives available in the current technology.
Applications such as traction very high speed drive drives requiring rapid acceleration drives requiring high maximum torques and large power rating drives which require variable) speed and leading power factor operation can only be effectively accomplished by the brush type machines.
The unique characteristics which are of interest to some specific applications are very high starting torques. constant and maximum torque over full speed range very high speeds. Infinitely variable speeds and characteristics offered by a series motors which makes it ideally suitable for traction application. These performance characteristics offered by the brush type machines are not obtained by other types of machines.
However, due to some serious limitations and disadvantages, which are well known &n the prior art such as brush wear carbon dust sparking and other maintenance and reliability related problems many attempts have made to replace the brush type with brushless machines.
One such replacement attempt is the variable frequency drive. In this drive a variable frequency voltage is applied lo an induction or synchronous machine. By varying the frequency of an input supply the speed of the motor is controlled over a wide range and variable speed operation is obtained. Although speed control on this type of machine is satisfactory but due to synchronous or induction operation the starting torque is not sufficiently high. On larger drives, due lo the presence of the harmonics of the supply frequency the sinusoidal waveform of the input supply voltage is distorted. Another disadvantage of the variable frequency drive is the cost of electronics needed lo achieve a variable frequency control. One more disadvantage of the drive is there reduction in efficiency due to the electronics involved.
Another candidate drive is using an induction motor unit with phase wound rotor as an exciter where power output from the rotor is used to energize armature of a machine generally located on the same shaft and having a separate stator. This type of brushless arrangement has also many problems. The output voltage and frequency varies with the speed and operation near and above synchronous speed is not possible. Many attempts have been made lo overcome these problems and have only partially resolved at the expense of extra hardware, adding to the cost and reducing the efficiency. Most of the applications utilizing phase wound induction machine is general directed towards only providing excitation to synchronous machine. No attempt has been made to use phase wound induction machine to provide power for commutated AC or DC machine as in such an arrangement the size of the phase wound machine would become prohibitively large.
Another candidate application to provide a brushless operation is to use a DC machine armature to provide power to the rotor of a machine mounted on the same shaft. The major problem with this type of machine is that it does not work al zero speeds and works poorly al low speeds. In addition il requires complicated excitation mechanism of its own. Its use is primary limited to synchronous machine.
One more brushless machine widely used in the prior art is the permanent magnet rotor electrically commutated motor. Although it performs satisfactorily in small ratings for larger ratings the permanent magnet rotor becomes too large. It will also have serious problems with demagnetization during operation and the overall cost of the machine will not be economical Also on average these motors are much more costly compared to the other colors of identical ratings.
Over the years many efforts have been made to achieve true brushless operation. However, most of the machines developed were based on the concepts that were already in existence. As a result the brushless machines evolved over the years were basically improvements over the previous generation of machines based on the concepts that have not changed for decades. The performance characteristics obtained from these machines were not meeting the requirements of the applications rather the applications were modified to suit the requirements of the brushless drives. An example would be railroad traction. A brush type series motor provided with an inverter can provide both optimum traction performance and regenerative braking returning the brake energy lo the supply lines. The problems related to brushes however, makes an alternate traction drive a necessity.
A variable frequency drive consisting of an induction motor controlled by an electronic variable frequency voltage source is now being considered for future traction application. Compared to brush type series motor, this drive configuration is not cost effective and does not provide optimum performance however in absence of any other a brushless drive able to overcome the limitations of variable frequency drive the variable frequency drive is the chosen candidate drive system.
Another example stressing the need for a new brushless color concept is the automobile. The color and generator requirements in a automobile clearly require a brushless motor and a generator, which will greatly increase the over all reliability of an automobile. However, in this instance also due lo absence of a cost effective alternatively brush type motors and generators are still being used with a disadvantage of a lower reliability.
Rotatable transformers have been employed efficiently to transform power form a stationary device to a rotating member and vice versa for a number of different applications besides brushless machines. The prior art rotatable transformers have inherent limitations of unstable performance over a wide speed range because the frequency of the secondary member changes with the variations in speed affecting the performance of the rotatable transformer. A need for a rotatable transformer with stable performance over a wider speed range exists for a large number of applications. Whereby a performance and cost gains can be realized by employing a superior rotatable transformer.
Polyphase commutator motors in large power ratings have been very successfully used on variable speed applications primary because it provides variable speed operation at leading power factor. Leading power factor operation is critical al large power ratings for economical operation. The overall cost of the machine is also lower than that of a variable frequency drive. However, no brushless machine operating as polyphase commutated color exists in the prior art.
From the foregoing discussion it is evident that the existing brushless machines known in the prior art are not capable of providing superior and unique performance characteristics provided by the brush and commutator type AC and DC machines. Over the years, the requirements imposed on electrical machines have been increasingly becoming stringent due to rapid advances in all the areas of the technology where rotating electrical machines are employed. The numerous Problems posed by brush type machines are therefor becoming totally unacceptable A long standing need for a reliable and totally brushless electrical machines with superior performance characteristics has become critical.